1. Field of the Invention.
The present invention relates to printers, and more particularly to printers in which a hammerbank or other elongated printing member is driven in reciprocating fashion by a rotating cam.
2. History of the Prior Art.
It is known in the printer arts to provide a reciprocating elongated printing member such as a hammerbank adjacent a length of print paper to accomplish printing. An example of such a printer is provided by U.S. Pat. No. 3,941,051 of Barrus et al., PRINTER SYSTEM, issued Mar. 2, 1976 and commonly assigned with the present application. In the printer described in the Barrus et al. patent, an elongated hammerbank is driven in reciprocating fashion across the width of a print paper supported by an elongated platen. As the hammerbank undergoes its back and forth reciprocating movement across the print paper, magnetic hammer acutators mounted along the length of the hammerbank are selectively actuated to release or "fire" hammer springs associated therewith. This causes dot printing impact tips mounted on the fired hammer springs to impact the print paper through a length of ink ribbon to print dots on the paper. A counterbalancing assembly is driven in reciprocating fashion in an opposite, out-of-phase relationship with the hammerbank to minimize the vibrational motion that would otherwise result from the reciprocation of the hammerbank.
In the printer system described in U.S. Pat. No. 3,941,051 of Barrus et al., the elongated hammerbank and the counterbalancing assembly are driven in opposing reciprocating fashion by a single cam which engages opposite cam follower assemblies coupled to the hammerbank and to the counterbalancing assembly. The cam, which is configured to have opposite lobes thereon, is rotatably driven through a coupled flywheel by a drive motor. As the cam rotates, repeated movements of the lobes thereon past the cam follower assemblies produces reciprocating motion of the hammerbank and the counterbalancing assembly. The cam follower assemblies which couple the hammerbank and the counterbalancing assembly to the cam have compression springs which force and maintain cam follower pulleys at the ends of the cam follower assemblies in contact with the cam.
During startup of printers such as of the type described in the Barrus et al. patent having a cam driven hammerbank and counterbalancing assembly, considerable torque is required to move the cam past the point of peak resistance which is where the peaks of the opposite lobes engage the opposite cam follower pulleys and provide maximum compression of the springs within the cam follower assemblies. The startup torque requirements are such that the current required to produce the torque is typically as much as ten times greater than the current required to maintain steady state operation thereafter in which the hammerbank and the counterbalancing assembly are driven at a chosen constant speed. In many cases, a larger cam drive motor than that which would be required to maintain steady state operation, is required because of the high starting torque which must be produced to overcome initial cam resistance. This is disadvantageous, both from the standpoint of increased motor size and cost and from the standpoint of high current requirements and the power supplies needed to produce such currents.
Accordingly, it would be advantageous to provide an improved printer cam drive arrangement. It would be particularly advantageous to provide such an arrangement in which startup can be accomplished without the large motor currents normally required and without the need for the larger and more expensive motors often needed to meet the startup requirements.